Electrical control system for grinding mill



May 31, 1966 A. R. MacPHERsoN ETAL 3,253,744

ELECTRICAL CONTROL SYSTEM FOR GRINDING MILL Filed Sept. 22, 1964 3Sheets-Sheet l May 31, 1966 A. R. MacPHERsoN ETAL 3,253,744

ELECTRICAL CONTROL SYSTEM FOR GRINDING MILL May 31, 1966 A. R.M-LICPHERSON ETAL 3,253,744-

I'ILECTRICAL CONTROL SYSTEM FOR GRINDING MILL i-ilcd Sept. 22, 1964 3Sheets-Sheet 3 United States Patent O 3,253,744, ELECTRICAL CONTROLSYSTEM FOR GRINDING NULL Arthur R. MacPherson, Montreal, Quebec, Canada,and

Robert C. Meaders, Miiwaukee, Wis., assignors to Nordberg ManufacturingCompany, Milwaukee, Wis., a

corporation of Wisconsin Filed Sept. 22, 1964, Ser. No. 398,450 16Claims. (Cl. 222-56) This invention relates to a control system forcontrolling the rate 'of feed to a grinding mill, and has application toa dual-chambered grinding mill, and is a continuation in part ofapplication Serial No. 317,605, iiled October 2l, 1963, now abandoned.

A primary purpose of the invention is a control system and method of thetype described in which the feed is varied in accordance with the weightof the charge in the mill.

Another purpose is a method and apparatus of the type described,suitable for use in a dual-chambered grinding mill, in which the weightof each chamber has components determined by the charge in bothchambers.

Another purpose is a simple and reliably operable electri'cai controlsystem -for use with a rotating grinding mill.

Another purpose is a control system and apparatus of the type describedwhich is eifective to control the rate of feed rto both chambers of adual-chambered material reduction mill.

Another purpose is a control system of the type described in which boththe charge in the mill and the rate of feed to the mill are monitored toprovide control data.

Another purpose is a material reduction mill control system utilizing apower override to provide an additional monitor of the charge in themill.

Another purpose is a method of controlling a material reduction mill inwhich the charge in the mill is utilized in regulating the desired rateof feed to the mill.

Another purpose is a method of varying the feed rate to a materialreduction mill automatically so as to maintain a predetermined weight ofcharge in the mill.

Another purpose is a method of varying the 'feed in a dual chamberedmaterial reduction mill to either chamber so as to maintain diierentcharges in each grinding chamber.

Another purpose is a method of controlling the feed and/or charge so asto maintain the maximum throughput or grinding capacity to either orboth grinding compartments and the mill as a whole.

Other purp-oses will appear in the ensuing specification, drawings andclaims.

The invention is illustrated diagrammatically in the following drawingswherein:

FIGURE 1 is a diagrammatic illustration of a mill of the type described.

FIGURE 2 is a schematic illustration of the control system suitable foruse with the mill in FIGURE l.

FIGURE 3 is Ia more detailed electrical schematic illustrating the meansfor controlling the rate of feed to the mill in FIGURE l, and

FIGURE 4 is `a diagrammatic illustration of a further form of controlsystem.

The invention should not be limited to any particular type of grindingmill or material reduction mill, although the invention has lapplicationwith a mill of the type shown in co-pending application Serial No.229,334, tiled October 9, 1962. This mill has a pair of grindingchambers separated by a central discharge area, which is illustrated at10 in FIGURE 1. The two chambers are illustrated at 12a and 12b, andthey may be generally similar in size and shape. The mill may besupported on its trunnions 14a and 14b, with the trunnions being mountedon suitable strain gauges 16a 'and 16b, which are in turn positioned onconcrete supports or the like 18a and 18b. Feed troughs 20a and 20b maybe positioned at each end of the mill for introducing material to beground into the chambers. The present control system may fbe used with a-so-called autogenofus grinding mill in which the charge in the millgrinds itself. The oontrol system may also be used with the moreconventional ball or rod mills, in which steel balls or rods have beenadded to aid in material reduction.

The strain gauges 16a and 16b illustrated in FIGURE l may be of aconventional type and provide a resist- Iance or impedance variable withthe superimposed weight on the gauge. 'Ilhe placement of the straingauges may vary, and it is satisfactory to place the gauges at thetrunnion supports. When the strain gauges are so positioned, the totalweight superimposed on each gauge is made [up of two components, onecomponent ybeing from the chamber directly adjacent the strain gauge,and the other component being caused =by the opposite chamber. Thisplacement of the gauges provides a ratio of 3:1 between the weightcomponent caused by the charges in the chambers. For example, the straingauge at the lefthand side of FIGURE l may have a weight superimposed onit made up three-fourths by the charge in the left-hand chamber andone-fourth of the charge in the right-hand chamber. 'Ill-le oppositestrain gauge will have a corresponding weight relationship. 'Ilheinvention should not be limited to this particular weight ratio as fedto motors 24a and 24b which are utilized to control4 the rate of feed tothe feed chutes 20a and 20b. Normally, suitable conveyors will bepositioned above the feed chutes and the motors 24a and 24b will drivethese l conveyors. The rate at which the conveyors are driven maydetermine the rate at which material is supplied through the feed chutesand into the grinding chambers, or the feed rate may be determined byvibrating feeders or lche like.

Suitable potentiometers, or rheostats, or variable resistances areindicated at 26a and 261:. Assuming a 3:1 ratio between the Weightcomponents as described above, each potentiometer may have two variableresistances with one being three times as large as the other. The largerlor primary resistance component of potentiometer 26a will be fed tobridge 22a and the smaller component will be fed to bridge 22h. In likemanner the two resistance components of potentiometer 2Gb will beutilized in bridges 2221 and 22a. yIn effect, the smaller resistancecomponent from each potentiometer is used to cancel out or balance thesmaller or minor weight,

component in the strain gauge impedance. In other words, strain gauge16a will have a weight component due to the charge in each chamber. Theweight component from the right-hand chamber will be the smaller orminor component. The smaller impedance from potentiometer 2Gb will beused to cancel out the weight component from the right-hand chamber andso permit a balance between the setting of potentiometer 26a and thatportion of the impedance from strain gauge 16a which represents theweight in the left-hand chamber. In some applications, iixed resistancemay be used, rather than variable resistances.

FIGURE 3 illustrates a bridge circuit which may be utilized at 22a or2211 in the circuit of FIGURE 2. A suitable power supply 28 supplies aD.C. voltage between terminals 30 and 32 of the bridge circuit. Onearm-of the bridge contains impedance 34, and assuming this is theleft-hand bridge of FIGURE 2, this impedance will be equal to the largeimpedance of potentiometer 26a. Impedance 36 will be equal to the minorcomponent or the feedback component from potentiometer 26b. Impedance 38will be equal to three-fourths of the reading of strain gauger 16assuming that the 3:1 ratio between weight components, as specifiedabove, is used. Impedance 40 will be equal to one-fourth of the readingof strain gauge a. Impedances 38 and 40 together Will equal the combinedimpedance provided by strain gauge 16a.

Terminals 42 and 44 provide the output from the bridge circuit. Thesetwo terminals are connected through suitable lines to. an amplifierindicated generally at 46 with the amplifier being connected to adirectional sensitive relay 48. The relay 48 may include normally opencontacts 50 and normally closed contacts 52. If the direction of theoutput from a bridge circuit indicates an excess of charge in a chamber,contacts 52 will open, adding resistance 54 in circuit with resistance56 and drive motor 58. If the charge is too low, contacts 50 will close,removing resistance 56 to speed up motor 58.

FIGURE 4 illustrates a modified form of control system. A materialreduction mill 60 may be a dual-chambered mill with chambers 62a and62h. Trunnions 64a and 64b at each end of the mill may lbe supported inany satisfactory manner, with the supports each including a plurality,in this case three, electric strain gauges or the like 66a and 6617. Aseach side of the mill has an identical control system, only one-half orone portion of the control system will be described hereinafter. Itshould be understood however that each portion of the control systemwill operate in a similar manner.

The electric signals from strain gauges 66a are carried by wires 68 to acombination power supply and computation circuit 70. Volt meters or thelike 72 may 4be connected to the unit 70 so that the current from anyone of the strain gauges may be read to give an indication of the loadthereon. The unit 70 is effective to supply power to the strain gaugesso that the strain gauges, which may be Variable resistances, may thenprovide electric signals indicative of the super-imposed loads. The unit70 will take the signals from the various strain gauges and provide-outputs which are indicative of the charge or load in each of the millchambers. This may be accomplished by circuits of the type illustratedin FIGURES A2 and 3, or by other types of comparable circuits. Eachchamber will have a load component in the other chamber, and the circuitof unit 70 is eifective to provide output signals representing only thecharge in a particular chamber.

The output from unit 70 is carried by wire 74 to a control instrument 76whichmay be a periodic error integrating controller of the typeman-ufactured by the Ramsey Engineering Company, of St. Paul, Minnesota.The controller 76 will be set at a predetermined weight or charge whichis the desired charge in the mill for optimum operation. The unit 76will compare4 the desired charge, which may be in the form of anelectric signal or may be'otherwise, with the signal received over line74 which is indicative of the actual load in the mill chamber. Thecontroller 76 will integrate or average the diiference or error signalfor a predetermined but adjustable period of time. If at the end of thesample interval, the average error signal is zero, then the controllerwill have no output. If a positive or negative error integral exists atthe end of the sample interval,

then t-he controller will produce a series of step increaseor decreasesignals, proportional to the magnitude of the error integral.

The output from controller 76 will be passed over wire 78 to anautomatic set point changer 80 of the type manufactured by RamseyEngineering Company, of St. Paul, Minnesota. The set point changer 80controls the rate of feed to the mill chambers as described hereinafter.Initially, this unit is set for a predetermined rate of feed which willgive the desired charge or load in the mill chamber. In the event thereis an error signal `from the controller 76, then the set rate of feed atunit 80 will be changed by the error signal, The unit 80 normallyprovides an output over line 82 to a second integrating controller 84which is effective to compare the set rate of feed from the unit 80 withthe actual measured rate of feed from the conveyor scale measuringsystem 86.

The unit 86 is effective to weigh the material being fed over conveyor88 and through a feed chute 90 to the mill chamber. The signal fromweighing unit 86 is compared or integrated in controller 84 with thesignal from the set point changer 80. Again, the error signal isintegrated over a period of time so that minute or brief changes in therate of feed, which do not continue, will not affect the overall rate offeed. The output from controller 84 is fed over a line 92 to a motorizedrheostat assembly 94 which is effective to control the rate of-supply ofair or some other operating fluid from line 96 to line 98. The airsupply through line 98 controls a feeder assembly 100 which determinesthe amount of material fed'from hopper 102 onto conveyor 88. The feeder100 is pneumatically or hydraulically operated to control the volume ofmaterial discharged from the hopper `onto the conveyor. The inventionshould not be limited to la pneumatic or hydraulic system, as othercontrol means, for example electric, may be practical.

It is desirable to provide a further control for the overall system. Athermal converter 104 monitors the load on the motors (not shown)driving the mill 60. If the load on the motors is excessive, or is abovea predetermined set point, the thermal converter 104 will provide asignal over line 106 and line 108 to the set point changer 80. The setpoint changer will have its feed rate reduced until the load on themotors is again within limits. Normally the set point changer -80 willreduce its load in steps, although it would be a continuous type ofvariation.

The use, operation and function of the invention are as follows:

In the form of FIGURES l, 2 and 3, when the charge in each chamber ofthe mill is equal to the charge which has been determined to provideoptimum grinding, impedances 38 Iand 40 will be balanced by impedances34 and 36 and there will be no output from either bridge circuit and nosignal will be supplied to either of the amplifiers. However, if thecharge in the left-hand chamber becomes too great, for any one of anumber of reasons, impedance 38 will be greater than impedance 34, whichis its balancing impedance, and there will be an output from the bridgeto the amplifier 46. Contacts 52 will open and motor 58 will be sloweddown to ldecrease the rate of feed to the left-hand chamber. Thiscondition will continue until the charge in the left-hand chamber issufficient to provide a balancing of impedances. The operation of thebridge circuit for the righthand chamber is exactly the same. When thecharge in a particular chamber is either too great or too small, motor58 will be speeded up or slowed down to change the rate of feed to thechamber and this condition will continue until the charge in the chamberprovides equal irnpedances or until the charge is at its predeterminedoptimum level. The direction or polarity of the output from the bridgecircuit determines whether contacts 50 or 52 will be operated.

The charge in the mill may be made up of the material being ground aswell as any grinding media, for example balls or rods. As describedherein, e-ach of the strain gauge measuring means provides an impedanceor a resistance which is variable in accordance with the weightsuperimposed upon it. By the particular placement of the strain gaugesthere is a 3:1 ratio between weight components from the two chambers. Itshould be realized that other positions of the strain gauges willprovide other weight ratios. Also, the invention should not be limitedto providing impedances which vary in accordance with the weightsuperimposed upon a strain gauge. What is important is to vary anelect-rical parameter in accordance with Weight. -A voltage may vary, ora current may vary.

In the form shown in FIGURE 4, there is 'an electric signal developedwhich is indicative of the charge in each of the mill chambers. Each ofthese signals isutilized in a system for controlling the charge to aparticular chamber. The control system consists of two integrating oraveraging units, one of which develops an error signal relating thepredetermined or set charge in the mill and the actual charge in themill, while the other develops an error signal by comparing the actualrate of feed with the set rate of feed. The first of these controllersor integrating units provides an output signal which is indicative ofthe difference, over a predetermined sampling interval, of the actualcharge in the mill against the set charge and this output signal isutilized in changing the set or desired rate of feed to the mill. Boththe rate of feed to the mill and the actual charge in the mill aremonitored and the two monitoring signals are together utilized tocontrol the rate of feed. The power override of FIGURE 4 providesadditional control. When the power of the mill drive is excessive,indicating a possible charge overload, the set rate of feed will bedecreased to bring mill drive motor operation within limits.

Although the invention has been described in connection with adual-chambered grinding mill, particularly an autogenous grinding mill,the invention obviously has other applications. The mill need not beautogenous, but may utilize balls, rods or other grinding media. Also,the particular control circuits may be used on a mill having oneYgrinding chamber or more than two grinding chambers.

Whereas the preferred form of the invention has been shown and describedherein, it should be realized that there are many modifications,substitutions and alterations thereto within the scope of the followingclaims.

We claim:

1. A method of controlling the yfeed to a dual-chambered materialreduction mill including the steps of weighing one chamber, said weightbeing made up by a major component from the charge in the chamber beingweighed and a minor component from the charge in the other chamber,varying an electrical impedance in accordance with said weight, removingfrom the impedance that portion caused by the charge in the otherchamber, providing a reference electrical impedance variable with thedesired charge in the chamber being weighed, compa-ring said referenceimpedance and the adjusted Weight impedance and varying the rate of feedto the chamber being weighed until said impedances are equal.

2; In a system for controlling the material feed to an integraldual-chambered material reduction mill, variable electrical impedance`strain gauge means for supporting each chamber of the mill, means forcomparing the impedance of the strain gauge means supporting eachchamber with a reference electrical impedance, and means for varying thefeed to each chamber of the mill in accordance with any differencebetween its associated strain gauge impedance and the referenceimpedance.

3. The system of claim 2 further characterized in that each strain gaugeimpedance has a component caused bythe weight of the material in eachchamber of the mill, and means for cancelling out that component of avstrain gauge impedance caused by the Weight of the material in theopposite chamber in the mill.

4. The system of claim 2 further characterized in that said referenceimpedances are variable.

v5. In a system for controlling the material feed to an integraldu-al-chambered material reduction mill, variable electrical imped-ancestrain gauge means for supporting each chamber of the mill, each straingauge impedance having a component due to the weight of the material ineach chamber of the mill, a pair of reference impedances, one `for ea-chchamber, means for comparing each strain gauge impedance and a referenceimpedance including a pai-r of bridge circuits, each bridge circuitincluding a str-ain gauge impedance, the reference impedance associatedwith a particular chamber, and a portion of the other referenceimpedance, and means for varying the feed to each chamber of the mill inaccordance with the output from said bridge circuits.

6. A method of controlling the feed to a material reduction millincluding the steps of developing an electric signal indicative of thecharge in the mill, comparing Said `signal with a reference representingthe desired charge in the mill to produce an electric signalrepresenting the difference between the actual and desired charge in themill, developing an electric signal representa-tive of the rate of feedof material to the mill, comparing said lastnamed signal with areference representing the desired rate ofv feed to the mill to producean electric signal repl resenting the difference between the actual andth'e desired rate of feed to the mill, utilizing the signal represtingthe difference between the lactual and desired charge in the mill tocontrol the desired rate of feed `to the mill, and utilizing the signalrepresenting the difference between the actual and desired rate of feedto the mill to control the rate of feed to the mill.

7. The method of claim 6 further characterized in that the step ofcomparing the signa-l representing the actual char-ge in the mill withthe reference occurs over a predetermined interval, with the differencebetween the signal and the reference over this interval being averaged.

S. The method of claim 6 further characterized in that the step ofcomparing the signal representing the actual rate of feed with thereference occurs over a predetermined interval, with the differencebetween the s ignal and the reference 'being averaged over thislinterval.

9. The method of claim 6 further characterized by and including the stepof monitoring the load on the mill drive to provide an electric signalto vary the reference of the mill rate of feed when the powerconsumption of the mill drive exceeds a predetermined level.

10. A method of controlling the feed to a material lreduction milllhaving a plurality of chambers including the `steps of developing aplurality of electric s-ignals, each indicative of the charge in achamber, comparing each of said signals with a reference representingthe desired charge in each chamber to produce a plurality of outputsignals each of which represents the difference between the actual anddesired charge for a particular mill chamber, developing a plurality ofelectric signals, each representative of the rate -of feed of materialto a mill chamber, comparingeach of said las-t-named signals with areference representing the desired rate of feed'to each mill chamber toproduce a plurality of electric signals each of which represents thedifference between the actual and desired rate of feed to a millchamber, utilizing the signals representing the difference between theactual and desired charge in the mill chambers to control the desiredrate of feed to each mill chamber, and utilizing the signals4representing the difference 'between the actual and desired rate offeed to each mill chamber to con-trol the rate of feed to the millchambers.

11. In a system of control for a material reduction mill, means fordeveloping an electric signal indicative of the charge in the mill,means for comparingsaid signal with a reference representing the desiredcharge in the mill to produce an output signal representing thedifference between the actual and desired charge in the mill, a materialfeed system for the mill, means for developing an electric signalrepresentative `of the rate of feed of material to the mill, means forcomparing said last-named signal with a reference represen-ting thedesired rate Iof feed to the mill to produce an electric signalrepresenting the difference between the actual and the desired rate offeed to the mill, means for utilizing the signal representing thedifference between Ithe actual and desired charge in the mill to controlthe `desired rate of feed to the mill, and means for utilizing thesignal representing the diiference between the actual and desired rateof feed to the mill to control the material feed system.

12. The system of claim 11 further characterized in that the means forcomparing the signal representing the charge in the mill with areference includes means for integrating the resultant difference over apredetermined interval.

13. The system of claim 11 further characterized in that the means forcomparing the signal representing rate of feed to the mill with areference includes means for integrating the resultant difference over apredetermined interval.

14. The system of claim 11 further characterized by and including meansfor monitoring power consumption in the mill drive, and means,cooperating with said monitoring means, for changing the desired rate offeed to the mill when powe-r consumption exceeds a predetermined level.l

15. Th-e system of claim 11 further characterized in that the materialfeed system is pneum-atically operated.

16. A control system for a material reduction mill having a plurality ofchambers, means for developing a plural-ity of electric signals, each`indicative of the charge in a mill chamber, means for comparing eachsignal with a reference yrepresenting the desired charge in each chamberto produce a plurality of 'output signals each representing thedifference between the actual and desired charge for a particular millchamber, a material feed system for each mill chamber, means fordeveloping a plurality of electric signals each representative of therate of feed to a mill chamber, means for comparing each of saidlastnamed signals with a reference representing the desired rate of feedto each mill chamber to produce a plurality of output signals eachrepresenting the difference between the actual and desired rate of feedto a particular chamber, means for utilizing the signals representingthe difference between the actual and desired charge in the millchambers to con-trol the desired rate of lfeedto each mill chamber, andmeans for utilizing the signals representing the difference between theactual and desired rate of feed to the mill chambers.

References Cited by the Examiner UNITED STATES PATENTS 2,764,360 9/1956Podszus 241-34 2,766,939 10/ 1956 Weston.

FOREIGN PATENTS 227,782 9/ 1959 Australia. 1,160,190 7/1958 France.1,131,974 5/1962 Germany.

,RAPHAEL M. LUPO, Primary Examiner. HADD s. LANE, Examiner.

1. A METHOD OF CONTROLLING THE FEED TO A DUAL-CHAMBERED MATERIALREDUCTION MILL INCLUDING THE STEPS OF WEIGHING ONE CHAMBER, SAID WEIGHTBEING MADE UP BY A MAJOR COMPONENT FROM THE CHARGE IN THE CHAMBER BEINGWEIGHED AND A MINOR COMPONENT FROM THE CHARGE IN THE OTHER CHAMBER,VARYING AN ELECTRICAL IMPEDANCE IN ACCORDANCE WITH SAID WEIGHT, REMOVINGFROM THE IMPEDANCE THAT PORTION CAUSED BY THE CHARGE IN THE OTHERCHAMBER, PROVIDING A REFERENCE ELECTRICAL IMPEDANCE VARIABLE WITH THEDESIRED CHARGE IN THE CHAMBER BEING WEIGHED, COMPARING SAID REFERENCEIMPEDANCE AND THE ADJUSTED WEIGHT IMPEDANCE AND VARYING THE RATE OF FEEDTO THE CHAMBER BEING WEIGHED UNTIL SAID IMPEDANCE ARE EQUAL.